Polyesters represented by polyethylene terephthalate (PET), polybutylene terephthalate (PBT) and polyethylene naphthalate (PEN) are excellent in mechanical characteristics and chemical characteristics, and they are used in broad fields such as, for example, fibers for clothing and industrial materials, films and sheets for packaging, magnetic tapes, optics and so on, bottles which are blow-molded articles, casings of electric or electronic parts, and other engineering plastic molded articles depending on the characteristics of each polyester.
Recent market diversification has led attentions on copolymerized polyesters in which other glycol component is copolymerized with the aforementioned polyesters. In particular, copolymers of neopentyl glycol or 1,4-cyclohexane dimethanol are amorphous and have a high glass transition point, and are widely used in the film filed and so on. These copolymerized polyesters and methods for producing the same have been disclosed (see PTDs 1 to 5, for example).
One use application of the aforementioned copolymerized polyesters lies in the filed of films and sheets. In such a field, high transparency, little contaminants, and high degree of clarification of the film or sheet are very important characteristics, and it is an important issue how these characteristics are ensured.
Generally, polyester is produced by preparing an oligomer mixture by esterification or transesterification between a dicarboxylic acid and/or an ester-forming derivative thereof, and a diol and/or an ester-forming derivative thereof, and liquid-phase polycondensing the oligomer mixture in a vacuum with the use of a catalyst.
Conventionally, as a polyester polycondensation catalyst that is used during such polycondensation of polyester, antimony compounds, germanium compounds, or titanium compounds are widely used.
Antimony trioxide is a catalyst that is low-priced and has excellent catalytic activity. However, when antimony trioxide is used as a main ingredient, namely it is used in such an adding amount that practical polymerization speed is achieved, metal antimony precipitates at the time of polycondensation, causing occurrences of darkening and contaminants in the polyester.
As a catalyst other than antimony compounds that has excellent catalytic activity and is capable of giving a polyester not having the aforementioned problems, titanium compounds have been already brought into practical use. However, polyesters produced by using titanium compounds such as tetraalkoxy titanate are disadvantageously prone to be thermally degraded at the time of melt molding, and result in significantly colored polyesters.
For the circumstances as described above, there is demanded a polycondensation catalyst composed of a metal component other than antimony, germanium and titanium compounds, as a main metal component of the catalyst, which has excellent catalytic activity, and gives polyesters having excellent color hue and thermal stability and giving a molded article with excellent transparency.
As a novel polycondensation catalyst responding to the aforementioned demand, a catalyst system composed of an aluminum compound and a phosphorus compound is disclosed and notable.
Regarding a method for producing polyester by the polycondensation catalyst system, a preferred adding time of the polycondensation catalyst system is disclosed (see PTDs 6 to 8, for example).
Polyesters obtained by the polycondensation catalyst system have excellent color hue, transparency and thermal stability, and respond to the aforementioned demand. However, in polymerization of copolymerized polyester system, in particular, the characteristics of little contaminants and high degree of clarification are not satisfactorily achieved, and amelioration thereof is strongly demanded. Polyester produced by copolymerization of neopentyl glycol by the method of PTD 5 is disadvantageously accompanied by a large quantity of contaminants.